www-browser and x-www-browser being different things, this would be a bug in the LXDE metapackage?

It is indeed, yes,* but what's the fix: what should the metapackage
actually depend upon, given x-www-browser virtual package does not exist
yet?
* or arguably whichever concrete lxde package actually contains the
shortcut definition

lxde-core depends on www-browser already (Suggests, actually, but that's
irrelevant). So it might as well depend on this to-be-done
x-www-browser.

BTW if a package ships a .desktop file evoking x-www-browser, is it not
obliged to Depend on (rather than Recommend/Suggest) a package which
provides that alternative? Or would a TryExec field in the .desktop file
be enough to cover it?